The current source. The Active Current Source

Size: px

Start display at page:

Download "The current source. The Active Current Source"

Erika Anthony
6 years ago
Views:

1 V ref + - The current source Minimum noise euals: Thevenin Norton = V ref DC current through resistor gives an increase of /f noise (granular structure) Accuracy of source also determined by the accuracy of Output impedance mostly too low Use active V 4kT converter 37 The Active Current Source V ref Output impedance is What about the noise? 38

2 Nullor implemented by one CE stage V ref = -(V ref - V BE )/ Temperature behavior V BE found in (how to solve?) Output impedance: β f ro rout = r0 + + r + r + b π f >> r π and >> r b r out = ( β f + ) r 0 39 Plot of the output impedance vs r out r o 0.7(β f +) (β f +) V = kt = r = kt π β β f = 00, = 5 mv r π V =5V Lower β f reduces V but also r out Better nullor approximation helps (r o must be increased) 40

3 Noise behavior of active current source 4kTr b C S out = B r b C B 4kT( + ) + + (+ β f ) (+ r π ) (+ r π ) r π β f β f infinite 0 B 0 (4kT/) 4kT/ S out Only base shot noise remains 4 Plot of noise vs S out c /β f /β f r π f contribution of C plus 4kT/ euals B, then = β f g V = 5 V and S out = 4 B m Lowering β f for reducing V does not help also, as: S out = 8kT /V The lower V the higher the noise = B 4 3

4 Current source demo 5 V 0kΩ Q 0kΩ β f = 00 Switch open: Sout 4kT + 4kT + B v out n, nv / Hz = 33 dbµ V 5kΩ ma 0kΩ + V analyzer - Switch closed: S 0 + 4kT + S out C v out n, nv / Hz = dbµ V 43 Current sources in V circuits Output impedance: V r out r π V relatively small r out = VAF C eduction of the DC loop gain accuracy Does not need to cost bandwidth as pole also shifts Noise: V C Noise of the active part approximately doubles C 44 4

5 Saturating transistors B V BC V BE C V CE Transistor saturates if C / B < β F Collector-emitter voltage is the difference of V BE and V BC PTAT V CE = kt L NM b Bg + + C / β r ln C β For C / B = β f denominator euals zero V CE not determined V CE reduces for larger β r (saturation voltage reduces) V CE reduces for larger C / B (deeper saturation) B f 45 O QP nfluence of layout on V saturation () C B E electron at has a larger chance on recombination than electron at Bad for β r Take care of a large β r C B E Most of the electrons have a short way to go Less recombination β r increases General: Collector and emitter overlap should be as large as possible 46 5

6 The current mirror in out Mirror used for: (n) (m) biasing purposes current-gain stages buffering (cascode) Transfer : in out = m n Again nullor can be implemented by wire standard current mirror 47 Signal-to-noise ratio and dynamic range of a current mirror SN is the ratio of smallest and largest signal that can be processed at the same time D is ratio of smallest and largest signal that can be processed, not necessarily at the same time Noise power proportional to collector current ( C ) Signal power proportional to the suare of C ( C ) For C = na and B=0 khz P,S SN P signal S noise SN = 5dB D D 48 6

7 Peaking current source Derived from current mirror ln F HG S = KJ ln F H G K J S kt / d d C C = 0 for = C kt Q Q Source can be implemented by a resistor relatively low ohmic and accurate a broad resistor relatively high ohmic and not very accurate a small resistor What about PSS? 49 V = kt T = 300 K =50 Ω =00 Ω 50 7

8 Peaking current source vs Current mirror 50µA 8. kω 00µA 00µA kω 00µA 50 Ω Q p Q Q Q p 50 must be accurate 8k does not need to be accurate k must be accurate 5, : 70%, 00% 30% Peaking current source Current mirror 5 8

9 Conclusion/Summary Several configurations of voltage and current sources discussed Current level is the performance determining uality, not a V constraint Circuit design is hampered by V criterion other topologies 53 9

Homework Assignment 08

Homework Assignment 08 Question 1 (Short Takes) Two points each unless otherwise indicated. 1. Give one phrase/sentence that describes the primary advantage of an active load. Answer: Large effective resistance